Terahertz light means an electromagnetic wave, in general definition, having a frequency in the range of 0.1 to 10 THz (where 1 THz=1012 Hz), i.e., a wavelength in the range of 0.03 mm to 3 mm belonging to submillimeter-wave region to the far-infrared region.
The terahertz light is expected to be applied in a wide range of fields, ranging from basic science fields such as radio astronomy, materials science, and biomolecular spectroscopy, to practical fields such as security, information communication, environment, and medical care. For example, when carrying out active measurement in which terahertz light is irradiated on an object and an image of reflected light from or transmitted light through the object is measured, a substance that has not been visible heretofore becomes visible.
Use of the active measurement makes it possible to carry out an inspection of poisonous substances in an envelope, detection of explosive substances or dangerous substances in a bag or container, an inspection of foreign substances in foods, an inspection of a semiconductor chip, and the like. Moreover, use of the active measurement also enables an inspection of deterioration degree of works of art, medical applications such as cancer tests, monitoring of real-time moisture in a plant, an inspection of defects of the inside of exterior wall tiles of a space shuttle, and the like.
Techniques using this kind of active measurement by terahertz light are disclosed in Patent Literatures 1 to 3.
The technique disclosed in Patent Literature 1 includes using a semiconductor chip in which two-dimensional electron gas (to be described below) is formed at a constant position from the surface of the chip, and irradiating terahertz light on the semiconductor chip while applying a magnetic field to the semiconductor chip. The technique further includes measuring an electric current that flows through carbon nanotubes by the irradiation, thereby detecting the intensity and frequency of the weak terahertz light. Note that “two-dimensional electron gas” means electrons that move in the two-dimensional plane along the junction interface between a semiconductor and an insulator, or the junction interface between heterogeneous semiconductors. That is to say, the state in which electrons serving as carriers are distributed in the planar form is referred to as two-dimensional electron gas. Note that, in the specification, a heterogeneous semiconductor means a semiconductor of a kind different from the others, or a semiconductor of a structure different from the others, using an inversion layer or the like.
The technique disclosed in Patent Literature 2 includes allowing a graphene (to be described below) to adhere to the surface of a semiconductor chip having an oxide layer formed thereon, and irradiating terahertz light on the graphene while applying a magnetic field to the graphene. The technique further includes measuring an electric current that flows through the semiconductor chip by the irradiation, thereby detecting the intensity and frequency of the weak terahertz light. Note that “graphene” is an atomic monolayer of two-dimensional carbon crystal, and is capable of absorbing light in any energy state because the energy band gap is zero, thus being suited to absorption of light such as terahertz light or infrared light that has extremely low energy and passes through most of the semiconductors.
The technique disclosed in Patent Literature 3 includes irradiating terahertz light (with a wavelength of 4 μm to 10 mm) on an object, and detecting scattered light from an electrode which is an example of the object, as a signal by the scattered-light detector, thereby detecting foreign substances which are included on the surface of the electrode or within the electrode, e.g., foreign metals.